unikernel/duniverse/ppxlib/examples/simple-extension-rewriter/ppx_get_env.ml

@@ -0,0 +1,20 @@
+open Ppxlib
+
+let expand ~ctxt env_var =
+  let loc = Expansion_context.Extension.extension_point_loc ctxt in
+  match Sys.getenv env_var with
+  | value -> Ast_builder.Default.estring ~loc value
+  | exception Not_found ->
+      let ext =
+        Location.error_extensionf ~loc "The environement variable %s is unbound"
+          env_var
+      in
+      Ast_builder.Default.pexp_extension ~loc ext
+
+let my_extension =
+  Extension.V3.declare "get_env" Extension.Context.expression
+    Ast_pattern.(single_expr_payload (estring __))
+    expand
+
+let rule = Ppxlib.Context_free.Rule.extension my_extension
+let () = Driver.register_transformation ~rules:[ rule ] "get_env"